The present invention relates to a device and a method for apportioning a movement of a machine element, in particular of a machine element that can be moved by two or more drives along a drive axis of a machine tool or production machine.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Conventional machine tools and production machines are frequently equipped with a so-called redundant kinematic drives, which use at least two separate drives to move a machine element, for example a tool clamping device of the machine or a tool, along a drive axis.
FIG. 1 shows schematically a machine element driven along a single drive axis X to illustrate the principle of a redundant kinematic drive. As seen in FIG. 1, a first drive has two linear motors 3 and 4 that can guide a beam 5 along two vertical support columns 1 and 2 oriented parallel to the drive axis X. A second support column 6 is secured to the beam 5 and guides a second drive, for example a linear motor 7, which also moves parallel to the drive axis X. The movement directions of the individual drives 3, 4, 7 are indicated by arrows. Machine elements, which in the depicted embodiments are represented by a machine clamping device 8 and a tool 9, are attached to the drive 7. It will be understood that the machine of FIG. 1 can include additional drive axes, which are not essential for an understanding of the invention and have been omitted from FIG. 1 for sake of clarity.
If the tool 9 is to be moved along the drive axis X to a predetermined desired position value, then a decision has to be made how to apportion the required movement among the drives 3, 4, and 7. The first drive 3, 4 has to move a large mass due to the size of the linear motors 3 and 4 and is therefore unable to move fast, whereas the second drive 7 needs to move only small masses (i.e., the machine clamping device 8 and the tool 9) and can therefore move dynamically along the drive axis X. Accordingly, a dynamic, i.e., a high-frequency, movement of the machine element should be executed by the second drive system 7, whereas a less dynamic, i.e., a low-frequency movement should be performed by the first drive 3, 4. It will be understood that other types of direct drives or indirect drives can also be used instead of the linear motors 3, 4 depicted in FIG. 1.
FIG. 2 shows a conventional control system for apportioning the movement of a machine port along a drive axis of a machine tool or production machine. A controller 23 of the machine includes a computer 10 that computes a number of desired drive axis values xsoll for controlling the movement of the machine element. The desired drive axis values xsoll can be determined from operating parameters set by an operator. It will be understood that the controller 23 can include other functions and procedures, which are not important for an understanding of the invention and have been omitted from FIG. 2 for the sake of clarity.
The so determined desired drive axis values xsoll are then divided into a low-frequency component and a high frequency component, whereby the low-frequency component is determined by filtering the desired drive axis values xsoll with a low-pass filter 11, generating filtered desired drive axis values xsollg at the output of the low-pass filter 11, which describe the low-frequency component of the movement. The high-frequency component of the tool movement is then determined by subtracting the filtered desired drive axis values xsollg from the desired drive axis values xsoll with a subtracter 18, generating a difference value xsollΔ at the output of the subtracter 18. The filtered desired drive axis values xsollg are supplied as control input variables to a first controller 19, which generates output signals for controlling a converter 24 that supplies power to a first drive 21 representing the linear motors 3 and 4 shown in FIG. 1. The first drive 21 generates actual drive axis values xist1, which are fed back to the first controller 19.
Likewise, the difference xsollΔ is supplied as control input variable to a second controller 20, which controls a converter 25 that supplies power a second drive 22 representing the linear motor 7 shown in FIG. 1. The second drive 22 generates actual drive axis values xist2, which are fed back to the second controller 20.
The low-pass filter 11 depicted in FIG. 2 is typically implemented in modern devices that apportion a movement between drives, as, for example, a Tschebyscheff filter, a Bessel filter, a Butterworth filter, or as an elliptical filter. These conventional low-pass filters disadvantageously do not have a constant group delay time, so that the phase response does not fall or rise linearly with the frequency. FIG. 6 shows the amplification V and the non-linear phase response of an exemplary elliptical filter.
Due to the non-constant phase delay time of these filters, the filtered desired drive axis values xsollg, unlike the desired drive axis values xsoll, have different temporal delays. In conventional control systems, where the desired drive axis values xsoll are not delayed before being subtracted from the filtered desired drive axis values xsollg in the subtracter 18, the resulting difference xsollΔ still has a relatively large contribution from the low-frequency component of the movement. It should be noted that delaying the desired drive axis values xsoll before subtraction would likely not be advantageous, because the temporal delays of the desired drive axis values xsoll in the filter can vary.
FIG. 3 shows corresponding signals for an exemplary low-frequency sinusoidal movement with a superimposed sinusoidal movement at a higher frequency. Unlike the desired drive axis values xsoll, the filtered desired drive axis values xsollg include only low-frequency components. The difference xsollΔ includes low-frequency components of the movement in addition to the high-frequency component, which significantly increases the amplitude of the difference xsollΔ. Consequently, in practical applications, the travel range of the dynamically configured second drive has to be oversized, which increases its cost.
It would therefore be desirable to provide a device and a method for optimally apportioning the movement of a machine element that is driven by multiple drives along a drive axis of a machine tool or production machine.